SEALS AND COD
by Debbie MacKenzie, December, 2002
Late in 1998, during the “International Year of the Ocean,” Mi’kmaq Elder and Chief Charlie Labrador was asked by the “International Ocean Institute” to address a major scientific conference held in Halifax, Nova Scotia. The theme of the conference was “the crisis of knowledge.” Following is an excerpt from Charlie’s speech to the gathering of marine scientists:
“What you are telling me is you don’t know how to fish…You use the word ‘technology,’ but in my time there has been a decrease in everything…If something isn’t done soon, there will be no more time for any of us. There has to be something better than technology. It was sad a few years ago when our seals got the blame for taking the cod. It wasn’t their fault…those who harvest the earth’s resources must begin putting as much back into it as they remove…”
In recent years, seal populations have increased their numbers in the North Atlantic Ocean. This has occurred as commercial fish stocks have plummeted. In many minds there can be no argument: seal predation on fish is now at an excessive level, and is “preventing the rebuilding” of the fish stocks. The demands for increased killing of seals and the establishment of “seal exclusion zones” in Atlantic Canada are getting louder. (e.g. FRCC, 2002)
There are groups who protest against seal killing, mainly on the grounds that it is inhumane (e.g. Sea Shepherd Conservation Society, International Fund for Animal Welfare). While I agree with their arguments, the lines of reasoning seemingly can be too easily discounted by cold, “objective” science. Therefore, my focus in this paper is to try to add to the credible scientific arguments against killing seals.
In Atlantic Canada, seals are facing two major charges:
1. They are
eating too many fish, especially cod.
While the first charge is considered to be more serious, the common knowledge that seals also spread cod worm (and cod are becoming increasingly “wormy”) adds to their negative public image and contributes to public acceptance of the “necessity” of killing them.
It is argued here that both charges against the seals are unsubstantiated, and evidence suggests that the truth may be quite the reverse. Extremely poor physical condition in Atlantic cod today presents a problem that should be helped by an increase, rather than a decrease, in natural predation. Also, “spreading cod worms” appears to enhance zooplankton production, and thereby also works to the ultimate benefit of cod and other marine species.
My arguments in defense of seals:
Charge #1. Seals are eating too many fish, specifically cod in Atlantic Canada.
It seems intuitive to many of us that low numbers of our preferred fish might be because seals are eating “too many” of them. This, however, has been very difficult to prove scientifically (Bonner, 1990). What effect does seal predation really have on the cod stock? One argument that has been offered is the observation that some of the fish eaten by seals would grow to be cod-consumers themselves, and that seals thereby spare the cod stock some degree of fish-predation that would naturally occur in the absence of seals. Interrelationships in the marine food web are so complex, that it becomes impossible to predict the effect on one species of changing the population of another (for example, see IFAW’s “simplified food web”).
But a stronger case can be made. A more direct way in which seal predation benefits cod, is by allowing better nutrition and growth in the surviving fish, after selectively thinning their numbers. It is an important observation that seal predation naturally enhances fish growth and health.
Cod growth rates in Atlantic Canada are now at historic lows.
Recognition of the reality that cod stocks in Atlantic Canada are now showing unmistakable signs of starvation, leads to the conclusion that “overpredation” by seals cannot be occurring. If it were, the remaining cod would be fatter. According to Bonner (1990), only a single case is known in which seal predation caused depletion of a fish population, and in that instance “The fish population showed all the signs of sustained severe depletion, i.e… fast growth rate…” (Bonner, 1990) In contrast, Atlantic cod (and other ground fish) have experienced dramatic slowing of growth rates and decline in body “condition” in recent decades…strongly suggesting population control by starvation rather than predation.
An appropriate level of natural predation wards off a scenario in which large numbers of fish inevitably die of starvation. A fish population facing starvation can therefore be expected to benefit from increased predation…so, rather than eating “too many cod,” it seems that natural predators may not be removing enough of them at present to ensure the health of the survivors. (Granted, there are low absolute numbers of cod in the ocean now, but marine production appears to have been reduced so far that even those few cod are not well fed. The situation is grim.)
Two important notes for fishery managers:
1. The fact that there are more seals living in our coastal waters now than there were a few decades ago, does not in itself lead to the conclusion that total natural predation pressure on immature cod has increased. This is because other predators that also consumed young cod are no longer physically in the picture, including large numbers of mature cod, halibut, and other fish that once grew large enough to catch the same small cod that seals eat. To conclude that natural predation on small cod has increased overall, it would first be necessary to quantify the extent to which small cod predation has decreased due to the generalized relative absence of large fish in the ocean today. This has not been done.
Does the recent increase in fish consumption by seals outweigh the recent decrease in fish consumption by other fish? Who knows? My guess is “No.” (From the point of view of the seals, the competition for small fish has decreased. And from the point of view of the cod stock, the fish now increasingly relies on the seals, rather than larger fish, for thinning their numbers by natural predation. We may mistakenly perceive this change as an “ecosystem imbalance” but it is more likely just a natural way that the ancient multi-species network of marine life works to re-establish equilibrium.)
2. Number comparisons can be deceptive. For example, regarding 4VsW cod, the FRCC has quoted DFO (in FRCC.2002.R.1):
“The models of cod consumption by grey seals imply a range from 5,400t to 22,000t of cod being removed by seals. These are relative to estimated biomass of 32,000t to 37,000t respectively.”
A person with no understanding of population dynamics might get the impression on reading this that the grey seals are eating a dangerously high fraction of the total cod population, and in fact that they might easily consume all of the fish within the next few years. But a direct comparison of these numbers is meaningless because the consumption by seals is an annual total, while the biomass estimate is a simple snapshot of the standing stock of cod at a given time. When considering the impact of a particular biomass removal route on an annual basis (such as seal consumption), at the very least this needs to be compared to the total biomass that is annually added to the stock (through production of juveniles and growth of individuals). The addition and subtraction of “cod biomass” at the younger ages normally occurs at a much greater rate than at the older ages. And the tonnage that flows in and out of the youngest portion of the cod stock on an annual basis is undoubtedly far greater than any single, static “biomass estimate.” Young cod, especially the very youngest, are normally fast growing but short lived, transient members of the stock. Biomass gains and losses happen quickly, and this is normal. The removal of a given tonnage of juveniles simply does not have the same implication for the population as the removal of a similar tonnage of mature adults. This is another unfair bias in the common comparisons of seal consumption to fishery removals.
Charge #2. Seals are damaging cod stocks by spreading the “cod worm” (Pseudoterranova decipiens, also sometimes called the “sealworm”).
The presence of the cod worm devalues cod fillets in the marketplace, but what role does this organism play in nature? The known facts include these:
- Cod worm has a complex life cycle, involving the development of larval stages in a series of cold-blooded hosts, including cod, with the adult stage ultimately living and reproducing in the guts of seals, especially the grey seal. Worm eggs are shed into the water via seal feces. (Illustration below, right, reproduced from Bonner's "The Natural History of Seals" - click on image to enlarge.)
- According to Bonner, cod worms (and other similar species) are “universal in the pinnipeds” (seals) and apparently do not cause the animals any significant harm. Bonner reports seeing “grey seals in excellent condition and with ample blubber reserves whose stomachs contained more than two litres of large nematodes.” (Bonner, 1990, p 133)
- In the cod, the larvae encyst in the muscles, where they remain dormant until the fish is eaten by the final host, the seal. This does not appear to place any great stress on the cod. In the warm stomach of the seal the larvae emerge from the cysts, finally mature and produce eggs.
- “The mature worms produce huge quantities of eggs which are shed into the water in the feces of the seal and hatch within 10-60 days, depending on water temperature.” (Bonner, 1990)
This scenario, of vast numbers of invertebrate eggs being regularly released into the ocean by seals…is this essentially a positive or a negative thing?
Beyond Bonner’s statement that the mature worms “produce huge quantities of eggs,” I cannot see where the reproductive output of the cod worm has been quantified. But it may be considerable. Many mature marine invertebrates divert significant energy and nutrient resources to spawn production. (Molluscs, for example, commonly have “gonadosomatic indices” in excess of 100%, meaning that the weight of eggs produced exceeds the weight of the animal itself. Incidentally, this index is much lower in fish species.)
In the stable, warm, nutrient-rich environment of the seal’s stomach, the reproductive output of the cod worm may well be far higher than that of similar non-parasitic worms. Exempt from the common requirements of searching for food and defending oneself against predation, this organism seems to have little to do besides produce eggs. The marine worm species that have evolved to take advantage of the guts of marine mammals may therefore play a greater ecological role than others in the replenishment of the zooplankton by spawn production.
Elsewhere I have argued that all spawn-producing marine species, fish and invertebrates, contribute to zooplankton maintenance in this way, because their pelagic eggs and larvae ARE zooplankton, and they also provide food for major zooplankton species such as copepods. The vast majority of the eggs produced are consumed as food by these tiny organisms. In this way, all these spawn producers contribute to the enhancement of marine production. It now becomes evident that marine mammals such as seals also perform this ecological service, making direct contributions to zooplankton maintenance by hosting marine worms and shedding their live spawn into the water. In today’s precarious situation of declining abundance of marine zooplankton, this steady output of worm eggs may represent an especially valuable ecological contribution.
“…those who harvest the earth’s resources must begin putting as much back into it as they remove…”
How well does the seal heed Charlie Labrador’s advice?
Seals do “harvest” resources from the sea. They live by consuming a variety of marine fish and invertebrates. Size of prey seems most significant. Most codfish consumed, for instance, are small, 20-30 cm in length, and 1-3 years old.
How do seals “put back” resources to replace what they have removed? It can be argued that seals completely repay the ecosystem debt that they incur by eating fish (…and even that they may repay this debt with interest).
- Seals excrete urine intermittently into the water, directly fertilizing phytoplankton growth (“primary production").
- Seals excrete nutrient-rich feces, which are consumed by benthic invertebrates, many of whom produce millions of excess pelagic spawn which enrich the zooplankton (“secondary production“).
(Unlike humans, who may harmfully add the concentrated urine and fecal waste of hundreds of thousands of mammals to estuaries, seals distribute their bodily wastes into coastal waters in a pattern that can easily be handled by the ecosystem. Seals could not tolerate the crowding that would be necessary for them to become sewage “polluters” in marine systems.)
- Seals excrete large numbers of live worm eggs into the water, directly enhancing zooplankton production.
- Seals (and other natural predators) cull fish stocks in a way that enhances fish health, by selectively removing the weakest individuals, and reducing fish numbers, thereby lessening the competition for food experienced by the fish.
- Natural death of seals results in the entire body being recycled as it is consumed by living marine organisms. The natural life cycle of a seal therefore effectively “removes” nothing from the ocean, while the fact of its existence acts to actively sustain and enhance marine production.
The contrasts with the ecosystem effects of human fishing become obvious. Millions of years ago a workable balance was established between seals, fish, worms, zooplankton, etc., and it is folly for us to think that we can “sustainably manage” fish production better than they can.
Finally, since the worry is still there in some minds that seals may yet ‘inadvertently’ harm the fish stocks now that human fishing has so altered the ecosystem, it bears mentioning that negative feedback mechanisms naturally exist in the ocean to prevent this. The worry is unfounded. For example:
- “Size refuge”: If adequately fed, some cod will grow too large to be vulnerable to seal and other natural predation. These large spawning fish are major contributors to maintaining their own species, and to enhancing zooplankton stability.
- If seals do ‘overeat,’ fish will grow more quickly as a result, attaining the “size refuge” (and ability to avoid predation) more quickly. Therefore, if seals eat too many cod, they will find cod becoming harder to catch.
- If fish stocks cannot support the seal population, the reproductive success and health of seals will decline, and their numbers will fall. (And there are already indications that this is occurring. (DFO, 1999))
To the dismay of human fish consumers, the worm infestation in Atlantic cod is increasing. What does this mean? Does it mean that seals are causing a problem for cod?
Not likely, although a rise in the size of the worm population, along with the seal population, is probably implied. Another factor favoring the successful completion of the worm’s life cycle is the fact that small prey-fish for cod are now scarce, which forces adult cod to rely more heavily on potentially worm-harboring benthic invertebrates. The more that a cod relies on these invertebrates, the more worms it will inevitably ingest. Therefore, the lowering of ecosystem productivity in itself seems to enhance the production of cod worms…which seem likely to be especially efficient spawn/zooplankton producing organisms…a turn of events which can be seen to function as a systemic compensatory response.
Subtle, stabilizing feedback loops, such as this one, seem certain to have been built into the marine ecosystem. (This is “Gaia’s” way…) For another example: declining zooplankton abundance will result in a greater fraction of phytoplankton ultimately sinking to the bottom, which then stimulates increased growth of benthic phytoplankton consumers, such as scallops, which are particularly prolific/efficient spawn producers (i.e. zooplankton rebuilders), in comparison to fish, for instance. Under a lowered marine productivity regime, such as we see today, the ocean naturally tends to divert relatively more production into these more efficient benthic ‘zooplankton producers’ and less into fish. This trend is reflected in recent marine fishery changes in many areas - for example, Georges Bank now produces far fewer fish than it did in the past, but scallop growth there is accelerating. Such multi-species stabilizing feedback mechanisms have undoubtedly contributed to the long-term success of the current marine species assemblage.
The zooplankton growth facilitated by one warm-blooded seal excreting worm spawn…this is not a lot when you consider the size of the ocean and the dimensions of the problem, and this one pathway doesn’t offer much hope for the “rebuilding” of the zooplankton stocks (which needs to be considered more urgently than that of the fish stocks). However, the marine production required to support one seal is not large either. My hunch (supported by the evidence of millions of years of coexistence with healthy cod stocks) is that the give and take done by the seal is nicely balanced…and that if there is any “net” effect on the ecosystem from it’s having existed, that the effect of the seal is a positive one.
The zooplankton growth facilitated by one live seal, although small, remains a step in the right direction. Fishery managers need to wake up to the concept of ‘zooplankton conservaton and protection,’ an idea which may help them to appreciate the natural roles of mature fish, seals, etc., in the maintenance of all marine life. The prudent decision, or to use today’s jargon, “the precautionary approach,” must now be to leave the seals alone, along with the fish and the other surviving marine organisms. Marine ecosystem productivity is dropping, this is very serious, and we do not yet appreciate all of the ways in which the ancient interdependent marine species naturally support one another. We just need to trust that they do…and be thankful that seals still survive.
“What is man without beasts? If all the beasts were gone, man would die from a great loneliness of spirit. For whatever happens to the beasts, soon happens to man. All things are connected...Whatever befalls the earth befalls the sons of the earth. Man did not weave the web of life: he is merely a strand in it. Whatever he does to the web, he does to himself."
- Chief Seattle, 1854
See also: the Mysterious death by 'freezing' of
Newfoundland cod in a proposed "Seal Exclusion Zone." (added April, 2003)
The dedicated staff of the Eastern Shore Wildlife Rehabilitation Facility
tried unsuccessfully to nurture this baby...but at least they tried.
Bonner, W. Nigel. 1990. The Natural History of Seals. Facts on File, Oxford.
DFO, 1999. Proceeding of the National Marine Mammal Review Committee. CSAS Proceedings Series 99/14 - document online at http://www.ncr.dfo.ca/csas/csas/proceedings/1999/p99_14.pdf
FRCC.2002.R.1 January, 2002. 2002/2003 Conservation Requirements for Groundfish Stocks on the Scotian Shelf and in the Bay of Fundy (4VwX), in Sub-Areas 0, 2 + 3 and Redfish Stocks. - document online at http://www.dfo-mpo.gc.ca.frcc/2002/jan2002e.pdf
See also on this website:
“Fish Eggs - the Perfect Food?” and references therein regarding broad trends in zooplankton.
“The Downturn of the Atlantic Cod” and references regarding evidence of starvation in cod.
“The Barnacle Zone” for concrete physical evidence of the long-term decline in marine productivity.